Abstract:

Selective isoelectric whey protein precipitation and aggregation is carried out at
laboratory scale in a standard configuration batch agitation vessel. Geometric scale-up
of this operation is implemented on the basis of constant impeller power input per unit
volume and subsequent clarification is achieved by high speed disc-stack centrifugation.
Particle size and fractal geometry are important in achieving efficient separation while
aggregates need to be strong enough to resist the more extreme levels of shear that are
encountered during processing, for example through pumps, valves and at the
centrifuge inlet zone.
This study investigates how impeller agitation intensity and ageing time affect aggregate
size, strength, fractal dimension and hindered settling rate at laboratory scale in order to
determine conditions conducive for improved separation. Particle strength is measured
by observing the effects of subjecting aggregates to moderate and high levels of process
shear in a capillary rig and through a partially open ball-valve respectively. The protein
precipitate yield is also investigated with respect to ageing time and impeller agitation
intensity. A pilot scale study is undertaken to investigate scale-up and how agitation
vessel shear affects centrifugal separation efficiency.
Laboratory scale studies show that precipitates subject to higher impeller shear-rates
during the addition of the precipitation agent are smaller but more compact than those
subject to lower impeller agitation and are better able to resist turbulent breakage. They
are thus more likely to provide a better feed for more efficient centrifugal separation.
Protein precipitation yield improves significantly with ageing, and 50 minutes of ageing is
required to obtain a 70 - 80% yield of α-lactalbumin.
Geometric scale-up of the agitation vessel at constant power per unit volume results in
aggregates of broadly similar size exhibiting similar trends but with some differences due
to the absence of dynamic similarity due to longer circulation time and higher tip speed
in the larger vessel. Disc stack centrifuge clarification efficiency curves show aggregates
formed at higher shear-rates separate more efficiently, in accordance with laboratory
scale projections. Exposure of aggregates to highly turbulent conditions, even for short
exposure times, can lead to a large reduction in particle size. Thus, improving separation
efficiencies can be achieved by the identification of high shear zones in a centrifugal
process and the subsequent elimination or amelioration of such.

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